Loss Of Catalyst Activity Research Articles

Enzyme identification and development of a whole‐cell biotransformation for asymmetric reduction ofo‐chloroacetophenone

Chiral 1-(o-chlorophenyl)-ethanols are key intermediates in the synthesis of chemotherapeutic substances. Enantioselective reduction of o-chloroacetophenone is a preferred method of production but well investigated chemo- and biocatalysts for this transformation are currently lacking. Based on the discovery that Candida tenuis xylose reductase converts o-chloroacetophenone with useful specificity (kcat/Km=340 M(-1) s(-1)) and perfect S-stereoselectivity, we developed whole-cell catalysts from Escherichia coli and Saccharomyces cerevisiae co-expressing recombinant reductase and a suitable system for recycling of NADH. E. coli surpassed S. cerevisiae sixfold concerning catalytic productivity (3 mmol/g dry cells/h) and total turnover number (1.5 mmol substrate/g dry cells). o-Chloroacetophenone was unexpectedly "toxic," and catalyst half-life times of only 20 min (E. coli) and 30 min (S. cerevisiae) in the presence of 100 mM substrate restricted the time of batch processing to maximally ∼5 h. Systematic reaction optimization was used to enhance the product yield (≤60%) of E. coli catalyzed conversion of 100 mM o-chloroacetophenone which was clearly limited by catalyst instability. Supplementation of external NAD+ (0.5 mM) to cells permeabilized with polymyxin B sulfate (0.14 mM) resulted in complete conversion providing 98 mM S-1-(o-chlorophenyl)-ethanol. The strategies considered for optimization of reduction rate should be generally useful, however, especially under process conditions that promote fast loss of catalyst activity.

Effects of support composition and pretreatment on the activity and selectivity of carbon-supported PdCu nCl x catalysts for the synthesis of diethyl carbonate

The oxidative carbonylation of ethanol to diethyl carbonate (DEC) has been investigated on catalysts prepared by dispersing CuCl 2 and PdCl 2 on activated carbon and carbon nanofibers. The objectives of this work were to establish the effects of support structure and pretreatment on the dispersion of the catalytically active components and, in turn, on the activity and selectivity of the catalyst for DEC synthesis. At the same surface loading of CuCl 2 and PdCl 2, partially oxidized carbon nanofibers resulted in a higher dispersion of the active components and a higher DEC activity than could be achieved on activated carbon. Catalyst characterization revealed that nearly atomic dispersion of CuCl 2 and PdCl 2 could be achieved on the edges of the graphene sheets comprising the carbon nanofibers. Over oxidation of the edges or their removal by heat treatment of the nanofibers resulted in a loss of catalyst activity. The loss of catalyst activity with time on stream could be overcome by the addition of ppm levels of CCl 4 to the feed. While catalysts prepared with CuCl 2 alone were active, a fivefold increase in activity was realized by using a PdCl 2/CuCl 2 ratio of 1/20. It is proposed that the Pd 2+ cations interact with [CuCl 2] − anions to form Pd[CuCl 2] 2 complexes that are stabilized through dative bonds formed with oxygen groups present at the edges of the graphene sheets of the support. A mechanism for DEC synthesis is discussed, and a role for the Pd 2+ cations as part of this mechanism is proposed.

Syngas production by combined carbon dioxide reforming and partial oxidation of methane over Ni/α-Al 2O 3 catalysts

Ni/α-Al 2O 3 catalysts were found to be active in the temperature range 600∼900 °C for both CO 2 reforming and partial oxidation of methane. The effects of Ni loading, reaction temperature and feed gas ratio for the combination of CO 2 reforming and partial oxidation of CH 4 over Ni/α-Al 2O 3 were investigated. Catalysts of xwt%Ni/α-Al 2O 3 ( x= 2.5, 5, 8 and 12) were prepared by wet impregnating the calcined support with a solution of nickel nitrate. XRD patterns and activity tests have verified that the 5wt%Ni/α-Al 2O 3 was the most active catalyst, as compared with the other prepared catalyst samples. An increase of the Ni loading to more than 5 wt% led to a reduction in the Ni dispersion. In addition, by combining the endothermic carbon dioxide reforming reaction with the exothermic partial oxidation reaction, the loss of catalyst activity with time on stream was reduced with the amount of oxygen added to the feed.

A dynamic non-isothermal model of a laboratory intermediate temperature fuel cell using PBI doped phosphoric acid membranes

A dynamic two-dimensional non-isothermal model is presented to simulate the transient response of an intermediate temperature fuel cell using phosphoric acid doped polybenzimidazole PBI membrane. The influence of the electrode double layer was investigated when performing step changes in cell potential. For particular operating conditions and cell parameters used in this study, a current overshoot could be found when a step change was performed. This overshoot was caused by the delayed change of local oxygen concentration behind the change in potential. The overshoot could be removed by increasing the double layer capacitance. At these conditions the current density moved smoothly to the steady-state value without an overshoot. A dynamic polarization curve was also predicted for cell potential changes between 0.9 and 0 V at 5 mV s −1. The results showed that in a second cycle of operation the cell performance improved due to an increase of the MEA temperature, due to heat retained in the fuel cell body by excess heat produced during the first cycle. If the external heaters were turned off this excess heat was released much faster, and performance would fell gradually with time. An on–off controller was added to the model to achieve a better temperature control during the two cycles. The dynamic model was modified to account for degradation mechanisms, in particular phosphoric acid loss from the catalyst layers and platinum sintering in the cathode catalyst layer. It was found that degradation due to phosphoric acid loss was minor compared with degradation caused by catalyst activity loss during the first hours of continuous operation. After this first period (around 300 h) the Pt particles size tended to stabilize and the effect of phosphoric acid loss started to emerge.

Hydrothermal study of Pt–Sn-based SAPO-34 supported novel catalyst used for selective propane dehydrogenation to propylene

Catalyst's regeneration is unavoidable part during dehydrogenation. The hydrothermal treatment influence on the performance of Pt–Sn-based SAPO-34 supported novel catalyst, used for propane dehydrogenation to propylene is investigated in this study. The catalyst shows excellent stability for mild steaming (nitrogen mixed steam), during four consecutive runs (reaction–regeneration mode). On the other hand, Pt–Sn/ZSM-5 was largely affected on mild steaming due to severe dealumination. In order to get into mechanistic understanding, severe hydrothermal treatment was carried our using pure steam. The substation loss in catalyst activity was noted. Both fresh and severe hydrotreated catalysts were characterized by XRD, XRF, O 2-pulse analysis of coke, NH 3-TPD, IR spectrum of adsorbed ammonia, H 2-TPR, HR-TEM and XPS, to explore reasons for change in catalytic properties. The texture/topology is found stable. Changes in surface ensembles occur due to the loss of Sn, Al, formation of SnO x species and in particular Pt sintering. This leads Pt active sites (zeolite–SnO–Pt) to inactive sites (zeolite–Pt, zeolite–PtO–Sn, Pt–Sn alloy, etc.) formation and reduced catalyst activity. TEM micrographs and H 2-chemisorption analysis confirms Pt particles agglomeration and/or sintering. About 98% catalyst activity is recovered by re-dispersed Pt using chlorination technique and credit goes to hydrothermally stable support (SAPO-34).

Degradation of Reactive Black 5 dye using anaerobic/aerobic membrane bioreactor (MBR) and photochemical membrane reactor

Three different types of advance treatment methods were evaluated for the degradation of Reactive Black 5 (RB5). The performance of two stage anaerobic SBR–aerobic MBR, anaerobic MBR with immobilized and suspended biocells and an integrated membrane photocatalytic reactor (MPR) using slurry UV/TiO 2 system were investigated. The results suggest that, nearly 99.9% color removal and 80–95% organic COD and TOC removal can be achieved using different reactor systems. Considering the Taiwan EPA effluent standard discharge criteria for COD/TOC, the degree of treatment achieved by combining the anaerobic–aerobic system was found to be acceptable. Anew, Bacillus cereus, high color removal bacterium was isolated from Anaerobic SBR. Furthermore, when this immobilized into PVA–calcium alginate pellets, and suspended in the anaerobic MBR was able to achieve high removal efficiencies, similar to the suspended biocells system. However, the immobilized cell Anaerobic MBR was found to be more advantageous, due to lower fouling rates in the membrane unit. Results from slurry type MPR system showed that this system was capable of mineralizing RB5 dyes with faster degradation rate as compared to other systems. The reactor was also able to separate the catalyst effectively and perform efficiently without much loss of catalyst activity.

Demonstration and Characterization of Ni/Mg/K/AD90 Used for Pilot-Scale Conditioning of Biomass-Derived Syngas

A fluidizable, Ni/Mg/K/AD90(90% α-Al2O3) catalyst was evaluated in a pilot-scale study with oak-derived syngas for tar and methane reforming at 900 °C. The catalyst was operated in a semi-batch mode for ten individual reaction cycles, each separated by a regeneration protocol consisting of steaming followed by H2 reduction. During the experiment, subsequent reaction cycles showed a decrease in activity as indicated by decreased initial methane conversion. During each reaction cycle, H2S poisoning was the dominant deactivation mechanism, while coking was deemed to be minor in comparison. Catalyst characterization by XRD and TPR suggest that the loss of activity for subsequent reaction cycles coincides with the formation of a NiAl2O4 species that is not fully reduced under process conditions, which in turn results in a decreased number of potential metallic nickel (Ni0) sites available for hydrocarbon steam reforming. An increase in nickel crystallite size with time-on-stream was also observed using XRD, indicating that sintering may also play a role in loss of catalyst activity, although this is not considered the primary deactivation mechanism because activity loss was observed even when nickel crystallite sizes remained nearly constant.

H3Mo12O40P-Catalyzed One-Pot Synthesis of Amidoalkyl Naphthols

Solid H3Mo12O40P is used for an efficient one-pot synthesis of amidoalkyl naphthols using aromatic aldehydes, β-naphthol, and urea or amides in carbon tetrachloride. The catalyst was efficiently recovered from the reaction mixture and reused with negligible loss of catalyst activity. Ambient conditions, simple workup, and good yield are some of the striking features of the present protocol.

Effect of oxygen addition on catalytic performance of Ni/SiO2·MgO toward carbon dioxide reforming of methane under periodic operation

Abstract This work investigates the catalytic performance of an industrial steam reforming Ni/SiO2·MgO catalyst toward dry reforming of CH4 under periodic operation. The effects of cracking/regeneration temperatures and O2 addition during regeneration on the catalyst stability and activity were determined and various characterizations i.e. BET, SEM, XRD, and TPO were employed to relate the catalyst performance with its physical properties. It was found that, without O2 addition, the catalyst showed good stability at 650 °C but observed high deactivation at 750 °C due to the formation of encapsulating carbon. The addition of O2 along with CO2 can eliminate all deactivation at 750 °C and no significant loss of catalyst activity was observed for at least 12 cracking/regeneration cycles. The optimal performance for periodic operation was found at the condition with 5 min of the cracking step followed by 5 min of the regeneration step at 750 °C with CO2/O2 ratio of 7/3.

New sulfonic acid ion-exchange resins for the preesterification of different oils and fats with high content of free fatty acids

The preesterification of free fatty acid (FFA) containing vegetable oils by strong acid ion-exchange resins was studied. The catalysts were characterized by scanning electron microscopy (SEM) and ion-exchange capacity. Based on a simple kinetic model the rate constants were determined. The effect of the catalyst structure, particle size, stirring speed, oil properties and water removal method on the reaction rate was investigated. Calculation of the turn over frequencies (TOFs) revealed high catalyst activities. Excellent conversions were obtained even without water removal, due to an extractive esterification. During recycling durability tests a catalyst activity loss occurred. The deactivation mechanism was investigated and the exhausted catalysts were successfully regenerated. A catalyst fouling was not observed. The presented preesterification process facilitates the processing of low quality, high FFA containing oils to biodiesel via basic transesterification and might help to reduce the production costs.

Heteropoly acids as useful recyclable heterogeneous catalysts for the facile and highly efficient aza-cope rearrangement of N-allylanilines

Heteropoly acid phosphomolybdic acid (H 3Mo 12O 40P) has been found to be an excellent catalyst for aza-cope rearrangement of N-allylanilines. Various organic solvents and ionic liquids were tested as reaction media. Better results were obtained in ionic liquid reaction media in terms of reaction time and yield of the product. Catalyst could be efficiently recovered from the reaction and reused with negligible loss of catalyst activity even after two cycles. Short reaction time, ambient conditions, simple workup and high yield are some of the striking features of the present protocol.

Imobilização de enzimas em suportes cromatográficos: uma ferramenta na busca por substâncias bioativas

The development and characterization of bioreactors or IMER (immobilized enzyme reactors) as research tools are important in the scope of medicinal chemistry and constitute an alternative for the rational development of drugs. This approach does not require highly purified enzymes or a great amount of protein, but increase the enzymatic stability against heat, organic solvents and pH, without too much loss of catalyst activity. Immobilized enzyme reactors (IMER) can be used for the accomplishment of high efficiency screening on-line and, thus inhibitors can be quickly identified. Here, we emphasize the development of IMER by use of different methods of immobilization and chromatographic supports. Their applications, in different areas of research, are also fully discussed.

TEMPO Supported on Magnetic C/Co‐Nanoparticles: A Highly Active and Recyclable Organocatalyst

TEMPO was grafted on graphene-coated nanobeads with a magnetic cobalt core by using a general applicable "click"-chemistry protocol. The new heterogeneous CoNP-TEMPO emerged as a highly active catalyst for the chemoselective oxidation of primary and secondary alcohols using bleach as terminal oxidant. The outstanding stability of the C/Co nanoparticles enables the nanopowder to tolerate several TEMPO-mediated iterative oxidation reactions without any significant loss in catalyst activity. Furthermore, the excellent magnetic properties enable the rapid separation and quantitative recycling of CoNP-TEMPO out of the reaction mixture by simple magnetic decantation. The recovered nanoparticles can be subsequently reused without any further purification.

Theoretical Analysis of the Mechanism for the Oxidative Carbonylation of Toluene to p-Toluic Acid by Rhodium Complexes

Density functional theory has been used to investigate the mechanism and kinetics of the liquid-phase, oxidative carbonylation of toluene to p-toluic acid (C 7 H 8 + CO + 1/2O 2 → p-C 7 H 6 COOH + H 2 O) catalyzed by Rh(III) cations. In toluene solution containing trifluoroacetic acid and dissolved CO, Rh(III) is coordinated to three trifluoroacetate (TFA) anions and two CO molecules as Rh(CO) 2 (TFA) 3 . The oxidative carbonylation of toluene is initiated by the addition of toluene across one of the Rh-O bonds of Rh(CO) 2 (TFA) 3 to form (C 7 H 7 )Rh-(CO) 2 (TFAH)(TFA) 2 . The latter species undergoes isomerization and CO migration to produce (C 7 H 7 CO)-Rh(CO)(TFAH)(TFA) 2 , which then coordinates another molecule of CO. The mixed anhydride of toluic and tirfluoroacetic acid, C 7 H 7 C(O)O(O)CCF 3 and Rh(CO) 3 (TFA), are produced by reductive elimination from (C7H7CO)Rh(CO)2(TFAH)(TFA)2. Para-toluic acid is then formed by hydrolysis of C 7 H 7 C(O)O(O)CCF 3 . The proposed reaction mechanism explains many of the observations reported in our previous experimental work (Zakzeski, J. J.; Bell, A. T. J. Mol. Catal. A 2007, 276, 8) and, in particular, the effect of temperature on the ratio of p- to m-toluic acid, the effects of H 2 O and the partial pressure of CO on the loss of catalyst activity, and the effect of Rh concentration on the formation of a catalytically inactive Rh dimer species.

Spatially Resolved Electrode Diagnostic Technique for Fuel Cell Applications

A non-destructive, diagnostic technique for evaluating the extent of degradation in electrodes on membrane-electrode assemblies (MEAs) has been developed that can be applied to parts from application-scale proton exchange membrane fuel cells. The modes of electrode degradation that are examined include: catalyst activity loss, catalyst area loss, and electrode support structure damage through carbon corrosion. The technique incorporates a current distribution tool to enable spatially resolved analysis at a length scale of approximately 1 cm. The MEA parts are evaluated through a combination of cyclic voltammetry and polarization curves performed at conditions designed to expose degradation in one of the modes described above.

Studies on bimetallic catalyst behavior during naphtha reforming

Reforming of naphtha over Pt–Sn/Al2O3 and Pt–Re/Al2O3 catalysts has been investigated. Addition of nitrogen accelerated the catalyst deactivation process, enabling the study of activity decline and subsequent regeneration. Effect of presulfiding on the activity, selectivity and regeneration capacity of the catalyst was also examined. Presence of nitrogen promoted the dealkylation of C7 hydrocarbons, resulting in a higher initial benzene yield which declined with loss in catalyst activity. Sulfiding lowered the aromatics yield by blocking some of the active sites but deactivation rate declined, prolonging catalyst life between regenerations. Performance of Pt–Sn catalyst was similar to that of Pt–Re catalyst except that the aromatics yield was somewhat higher in the latter case.

Effects of hydrogen on carbon nanotube formation in CH 4/H 2 plasmas

Carbon nanotubes (CNTs) were synthesized using CH 4/H 2 plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH 4/H 2 = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H 2 supply to the CH 4/H 2 mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH 4/H 2 = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H 2 plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H 2 plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C–H bonds were produced in CNTs during their growth.

Coke formation on WO3/SiO2 metathesis catalysts

An appreciable amount of coke forms over an 8 wt% WO3/SiO2 during the metathesis of 1-alkenes to internal alkenes. Catalyst activity is maintained despite the high coke levels (49 wt%). To understand this phenomenon, the coked catalyst was characterized by thermogravimetric analysis (TGA), BET and energy filtered transmission electron microscopy (EFTEM). Even at high coke levels, carbon maps show that the coke is dispersed over the support rather than covering the tungsten oxide. This observation may explain why the high activity of the catalyst is maintained and gives an indication that the location of coke laydown is more important than the amount deposited, in determining catalyst activity. Trace quantities of water, butanol and 2-pentanone were introduced into the feed stream and this resulted in the inhibition of coke formation without a significant loss in catalyst activity. 2-Pentanone was found to be the most effective inhibitor. Coking profiles indicated that the internal alkenes coked more rapidly than the 1-alkene feed.

A new thermoregulated PEG biphasic system and its application for hydroformylation of 1-dodecene

The development of a new thermoregulated polyethylene glycol (PEG) biphasic system composed of PEG-400/1,4-dioxane/ n-heptane is reported in this paper. By applying this system in the hydroformylation of 1-dodecene catalyzed by Rh complexes modified with phosphite ligand containing PEG chains, TMPGP (P[O(CH 2CH 2O) n CH 3] 3, n = 8), the conversion of 1-dodecene and the yield of aldehydes reached up to 96% and 94%, respectively. The catalyst, after being recycled twenty-three times, showed no appreciable loss of catalyst activity. The average Rh loss was 0.65% each run.

Effect of open circuit voltage on performance and degradation of high temperature PBI–H 3PO 4 fuel cells

The impact of open circuit voltage (OCV) on the performance and degradation of polybenzimidazole–phosphoric acid (PBI–H 3PO 4) fuel cells operated at 180 °C was investigated. The OCV showed an initial quick increase in the first few minutes, followed by a much slower increase, and peaked after about 35 min. It then started an exponential and monotonous decline. Along with the decline of OCV, the performance of the fuel cell also declined. Operating the fuel cell with a load of 0.2 A cm −2 could temporarily boost the OCV and the fuel cell performance, but it could not recover the lost performance permanently. Electrochemical impedance spectroscopy (EIS) indicated significant loss of catalyst activity and increase in mass transport resistance due to the relatively high potential at OCV. X-ray diffraction (XRD) measurement showed that the cathode Pt crystallite size increased by as much as 430% after a total of 244.5 h of exposure to OCV.